Process for recovering alkali metals and sulfur from alkali metal sulfides and polysulfides

ABSTRACT

Alkali metals and sulfur may be recovered from alkali monosulfide and polysulfides in an electrolytic process that utilizes an electrolytic cell having an alkali ion conductive membrane. An anolyte solution includes an alkali monosulfide, an alkali polysulfide, or a mixture thereof and a solvent that dissolves elemental sulfur. A catholyte includes molten alkali metal. Applying an electric current oxidizes sulfide and polysulfide in the anolyte compartment, causes alkali metal ions to pass through the alkali ion conductive membrane to the catholyte compartment, and reduces the alkali metal ions in the catholyte compartment. Liquid sulfur separates from the anolyte solution and may be recovered. The electrolytic cell is operated at a temperature where the formed alkali metal and sulfur are molten.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/781,557, filed Mar. 14, 2013, which is incorporatedby reference. This application is a continuation-in-part of U.S.application Ser. No. 12/576,977, filed Oct. 9, 2009, which claims thebenefit of U.S. Provisional Patent Application No. 61/103,973, filedOct. 9, 2008.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under Award No.DE-FE0000408 awarded by the United States Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a process for removing nitrogen,sulfur, and heavy metals from sulfur-, nitrogen-, and metal-bearingshale oil, bitumen, heavy oil, or refinery streams. More particularly,the invention relates to a method of regenerating alkali metals fromsulfides (mono- and polysulfides) of those metals. The invention furtherrelates to the removal and recovery of sulfur from alkali metal sulfidesand polysulfides.

BACKGROUND OF THE INVENTION

The demand for energy and the hydrocarbons from which that energy isderived is continually rising. The hydrocarbon raw materials used toprovide this energy, however, contain difficult to remove sulfur andmetals that hinder their usage. Sulfur can cause air pollution, and canpoison catalysts designed to remove hydrocarbons and nitrogen oxide frommotor vehicle exhaust. Similarly, other metals contained in thehydrocarbon stream can poison catalysts typically utilized for removalof sulfur through standard and improved hydro-desulfurization processeswhereby hydrogen reacts under extreme conditions to break down thesulfur bearing organo-sulfur molecules.

Extensive reserves of shale oil exist in the U.S. that will increasinglyplay a role in meeting U.S. energy needs. Over 1 trillion barrelsreserves lay in a relatively small area known as the Green RiverFormation located in Colorado, Utah, and Wyoming. As the price of crudeoil rises, the resource becomes more attractive but technical issuesremain to be solved. A key issue is addressing the relatively high levelof nitrogen contained in the shale oil chemistry after retorting as wellas addressing sulfur and metals content.

Shale oil characteristically is high in nitrogen, sulfur, and heavymetals which makes subsequent hydrotreating difficult. According toAmerica's Strategic Unconventional Fuels, Vol. III—Resource andTechnology Profiles, p. 111-25, nitrogen is typically around 2% andsulfur around 1% along with some metals in shale oil. Heavy metalscontained in shale oil pose a large problem to upgraders. Sulfur andnitrogen typically are removed through treating with hydrogen atelevated temperature and pressure over catalysts such as Co—Mo/Al₂O₃ orNi—Mo/Al₂O₃. These catalysts are deactivated as the metals mask thecatalysts.

Another example of a source of hydrocarbon fuel where the removal ofsulfur poses a problem is in bitumen existing in ample quantities inAlberta, Canada and heavy oils such as in Venezuela. In order to removesufficient sulfur from the bitumen for it to be useful as an energyresource, excessive hydrogen must be introduced under extremeconditions, which creates an inefficient and economically undesirableprocess.

Over the last several years, sodium has been recognized as beingeffective for the treatment of high-sulfur petroleum oil distillate,crude, heavy oil, bitumen, and shale oil. Sodium is capable of reactingwith the oil and its contaminants to dramatically reduce the sulfur,nitrogen, and metal content through the formation of sodium sulfidecompounds (sulfide, polysulfide and hydrosulfide). Examples of theprocesses can be seen in U.S. Pat. Nos. 3,785,965; 3,787,315; 3,788,978;4,076,613; 5,695,632; 5,935,421; and 6,210,564.

An alkali metal such as sodium or lithium is reacted with the oil atabout 350° C. and 300-2000 psi. For example 1-2 moles sodium and 1-1.5moles hydrogen may be needed per mole sulfur according to the followinginitial reaction with the alkali metal:

R—S—R′+2Na+H₂→R—H+R′—H+Na₂S

R,R′,R″—N+3Na+1.5H₂→R—H+R′—H+R″—H+Na₃N

Where R, R′, R″ represent portions of organic molecules or organicrings.

The sodium sulfide and sodium nitride products of the foregoingreactions may be further reacted with hydrogen sulfide according to thefollowing reactions:

Na₂S+H₂S→2 NaHS (liquid at 375° C.)

Na₃N+3H₂S→3 NaHS+NH₃

The nitrogen is removed in the form of ammonia which may be vented andrecovered. The sulfur is removed in the form of an alkali hydrosulfide,NaHS, which is separated for further processing. The heavy metals andorganic phase may be separated by gravimetric separation techniques. Theabove reactions are expressed using sodium but may be substituted withlithium.

Heavy metals contained in organometallic molecules such as complexporphyrins are reduced to the metallic state by the alkali metal. Oncethe heavy metals have been reduced, they can be separated from the oilbecause they no longer are chemically bonded to the organic structure.In addition, once the metals are removed from the porphyrin structure,the nitrogen heteroatoms in the structure are exposed for furtherdenitrogenation.

The following is a non-limiting description of the foregoing process ofusing alkali metals to treat the petroleum organics. Liquid phase alkalimetal is brought into contact with the organic molecules containingheteroatoms and metals in the presence of hydrogen. The free energy ofreaction with sulfur and nitrogen and metals is stronger with alkalimetals than with hydrogen so the reaction more readily occurs withoutfull saturation of the organics with hydrogen. Hydrogen is needed in thereaction to fill in the where heteroatoms and metals are removed toprevent coking and polymerization, but alternatively, gases other thanhydrogen may be used for preventing polymerization. Once the alkalimetal compounds are formed and heavy metals are reduced to the metallicstate, it is necessary to separate them. This is accomplished by awashing step, either with steam or with hydrogen sulfide to form ahydroxide phase if steam is utilized or a hydrosulfide phase if hydrogensulfide is used. At the same time alkali nitride is presumed to react toform ammonia and more alkali hydroxide or hydrosulfide. A gravimetricseparation such as centrifugation or filtering can separate the organic,upgraded oil, from the salt phase.

In conventional hydrotreating, instead of forming Na₂S to desulfurize,or forming Na₃N to denitrogenate, H₂S and NH₃ are formed respectively.The reaction to form hydrogen sulfide and ammonia is much less favorablethermodynamically than the formation of the sodium or lithium compoundsso the parent molecules must be destabilized to a greater degree for thedesulfurization of denitrogenation reaction to proceed. According to T.Kabe, A Ishihara, W. Qian, in Hydrodesulfurization andHydrodenitrogenation, pp. 37, 110-112, Wiley-VCH, 1999, thisdestabilization occurs after the benzo rings are mostly saturated. Toprovide this saturation of the rings, more hydrogen is required for thedesulfurization and denitrogenation reactions and more severe conditionsare required to achieve the same levels of sulfur and nitrogen removalcompared to removal with sodium or lithium. As mentioned above,desulfurizing or denitrogenating using hydrogen without sodium orlithium is further complicated with the masking of catalyst surfacesfrom precipitating heavy metals and coke. Since the sodium is in theliquid phase, it can more easily access the sulfur, nitrogen and metalswhere reaction is desirable.

Once the alkali metal sulfide has been separated from the oil, sulfurand metals are substantially removed, and nitrogen is moderatelyremoved. Also, both viscosity and density are reduced (API gravity isincreased). Bitumen or heavy oil would be considered synthetic crude oil(SCO) and can be shipped via pipeline for further refining. Similarly,shale oil will have been considerably upgraded after such processing.Subsequent refining will be easier since the troublesome metals havebeen removed.

Although the effectiveness of the use of alkali metals such as sodium inthe removal of sulfur has been demonstrated, the process is notcommercially practiced because a practical, cost-effective method toregenerate the alkali metal has not yet heretofore been proposed.Several researchers have proposed the regeneration of sodium using anelectrolytic cell, which uses a sodium-ion-conductive beta-aluminamembrane. Beta-alumina, however, is both expensive and fragile, and nosignificant metal production utilizes beta-alumina as a membraneseparator. Further, the cell utilizes a sulfur anode, which results inhigh polarization of the cell causing excessive specific energyrequirements.

Metallic sodium is commercially produced almost exclusively in aDowns-cell such as the cell described in U.S. Pat. No. 1,501,756. Suchcells electrolyze sodium chloride that is dissolved in a molten saltelectrolyte to form molten sodium at the cathode and chlorine gas at theanode. The cells operate at a temperature near 600° C., a temperaturecompatible with the electrolyte used. Unlike the sulfur anode, thechlorine anode is utilized commercially both with molten salts as in theco-production of sodium and with saline solution as in the co-productionof sodium hydroxide.

Another cell technology that is capable of reducing electrolyte meltingrange and operation of the electrolyzer to less than 200° C. has beendisclosed by Jacobsen et al. in U.S. Pat. No. 6,787,019 and Thompson etal. in U.S. Pat. No. 6,368,486. In those disclosures, low temperatureco-electrolyte is utilized with the alkali halide to form a lowtemperature melting electrolyte.

Gordon in U.S. Pat. No. 8,088,270 teaches the utilization of solventswhich dissolve sulfur at a cell operating temperature and dissolvingsodium polysulfide in such solvents to form an anolyte which whenintroduced into a cell with an alkali ion conductive membrane areelectrolyzed to form sulfur at the anode and alkali metal at the cathodeand where a portion of the anolyte is removed from the cell, allowed tocool until the sulfur precipitates out.

It is an object of the present invention to provide a cost-effective andefficient method for the regeneration of alkali metals used in thedesulfurization, denitrogenation, and demetallation of hydrocarbonstreams. As will be described herein, the present invention is able toremove contaminants and separate out unwanted material products fromdesulfurization/denitrogenation/demetallation reactions, and thenrecover those materials for later use.

Another objective of the present invention is to teach improvements inthe process and device for recovering alkali metal from alkali metalsulfide generated by the sulfur removal and upgrading process.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a process for removing nitrogen, sulfur,and heavy metals from sulfur-, nitrogen-, and metal-bearing shale oil,bitumen, heavy oil, or refinery streams. The present invention furtherprovides an electrolytic process of regenerating alkali metals fromsulfides, polysulfides, nitrides, and polynitrides of those metals. Thepresent invention further provides an electrolytic process of removingsulfur from a polysulfide solution.

One non-limiting embodiment within the scope of the invention includes aprocess for oxidizing alkali metal sulfides and polysulfideselectrochemically. The process utilizes an electrolytic cell having analkali ion conductive membrane configured to selectively transportalkali ions, the membrane separating an anolyte compartment configuredwith an anode and a catholyte compartment configured with a cathode. Ananolyte solution is introduced into the anolyte compartment. The anolytesolution includes an alkali metal sulfide and/or polysulfide and ananolyte solvent that partially dissolves elemental sulfur and alkalimetal sulfide and polysulfide. A catholyte solution is introduced intothe catholyte compartment. The catholyte solution includes alkali metalions and a catholyte solvent. The catholyte solvent may include one ofmany non-aqueous solvents such as tetraethylene glycol dimethyl ether(tetraglyme), diglyme, dimethyl carbonate, dimethoxy ether, propylenecarbonate, ethylene carbonate, diethyl carbonate. The catholyte may alsoinclude an alkali metal salt such as an iodide or chloride of the alkalimetal. Applying an electric current to the electrolytic cell oxidizessulfide and/or polysulfide in the anolyte compartment to form higherlevel polysulfide and causes high level polysulfide to oxidize toelemental sulfur. The electric current further causes alkali metal ionsto pass through the alkali ion conductive membrane from the anolytecompartment to the catholyte compartment, and reduces the alkali metalions in the catholyte compartment to form elemental alkali metal.

Sulfur may be recovered in the liquid form when the temperature exceedsthe melting point of sulfur and the sulfur content of the anolyteexceeds the solubility of the solvent. Most of the anolyte solvents havelower specific gravity compared to sulfur so the liquid sulfur settlesto the bottom. This settling may occur within a settling zone in thecell where the sulfur may be drained through an outlet. Alternatively aportion of the anolyte solution may be transferred to a settling zoneout of the cell where settling of sulfur may occur more effectively thanin a cell.

The melting temperature of sulfur is near 115° C. so the cell is bestoperated above that temperature, above 120° C. At that temperature orabove, the alkali metal is also molten if the alkali metal is sodium.Operation near a higher temperature, such as in the 125-150° C. range,allows the sulfur to fully remain in solution as it is formed from thepolysulfide at the anode, then when the anolyte flows to a settlingzone, within or external to the cell where the temperature may be 5-20°C. cooler, the declining solubility of the sulfur in the solvent resultsin a sulfur liquid phase forming which is has higher specific gravityand settles from the anolyte. Then when the anolyte flows back towardthe anodes where sulfur is forming through electrochemical oxidation ofpolysulfide, the anolyte has solubility has the capacity to dissolve thesulfur as it is formed, preventing fouling and polarization at theanodes or at membrane surfaces.

In one non-limiting embodiment within the scope of the invention, a cellfor electrolyzing an alkali metal sulfide or polysulfide is providedwhere the cell operates at a temperature above the melting temperatureof the alkali metal and where the cathode is wholly or partiallyimmersed in a bath of the molten alkali metal with a divider between ananolyte compartment and a catholyte compartment. In this case thecatholyte essentially comprises molten alkali metal but may also includesolvent and alkali metal salt. The divider may be permeable to alkalimetal cations and substantially impermeable to anions, solvent anddissolved sulfur. The divider comprises in part an alkali metalconductive ceramic or glass ceramic. The divider may be conductive toalkali ions which include lithium and sodium.

In another non-limiting embodiment, a cell for electrolyzing an alkalimetal polysulfide is provided with an anolyte compartment and acatholyte compartment where the anolyte solution comprises a polarsolvent and dissolved alkali metal polysulfide. The anolyte solutioncomprises a solvent that dissolves to some extent elemental sulfur. Theanolyte may comprise a solvent where one or more of the solventsincludes: N,N-dimethylaniline, quinoline, tetrahydrofuran, 2-methyltetrahydrofuran, benzene, cyclohexane, fluorobenzene, thrifluorobenzene,toluene, xylene, tetraethylene glycol dimethyl ether (tetraglyme),diglyme, isopropanol, ethyl propional, dimethyl carbonate, dimethoxyether, dimethylpropyleneurea, formamide, methyl formamide, dimethylformamide, acetamide, methyl acetamide, dimethyl acetamide,triethylamine, diethyl acetamide, ethanol and ethyl acetate, propylenecarbonate, ethylene carbonate, and diethyl carbonate.

In one non-limiting embodiment, a method for oxidizing sulfides andpolysulfides electrochemically from an anolyte solution at an anode isdisclosed where the anolyte solution comprises in part an anolytesolvent that dissolves to some extent elemental sulfur. In the method,the anolyte solvent that dissolves to some extent elemental sulfur isone or more of the following: N,N-dimethylaniline, quinoline,tetrahydrofuran, 2-methyl tetrahydrofuran, benzene, cyclohexane,fluorobenzene, thrifluorobenzene, toluene, xylene, tetraethylene glycoldimethyl ether (tetraglyme), diglyme, isopropanol, ethyl propional,dimethyl carbonate, dimethoxy ether, dimethylpropyleneurea, formamide,methyl formamide, dimethyl formamide, acetamide, methyl acetamide,dimethyl acetamide, triethylamine, diethyl acetamide, ethanol and ethylacetate, propylene carbonate, ethylene carbonate, and diethyl carbonate.

In another non-limiting embodiment, a cell for electrolyzing an alkalimetal monosulfide or a polysulfide is provided with an anolytecompartment and a catholyte compartment where the anolyte solutioncomprises a polar solvent and dissolved alkali metal monosulfide or apolysulfide. The anolyte solution comprises a solvent that dissolves tosome extent elemental sulfur. The anolyte may comprise a solvent whereone or more of the solvents includes: N,N-dimethylaniline, quinoline,tetrahydrofuran, 2-methyl tetrahydrofuran, benzene, cyclohexane,fluorobenzene, thrifluorobenzene, toluene, xylene, tetraethylene glycoldimethyl ether (tetraglyme), diglyme, isopropanol, ethyl propional,dimethyl carbonate, dimethoxy ether, dimethylpropyleneurea, formamide,methyl formamide, dimethyl formamide, acetamide, methyl acetamide,dimethyl acetamide, triethylamine, diethyl acetamide, ethanol and ethylacetate, propylene carbonate, ethylene carbonate, and diethyl carbonate.

In one non-limiting embodiment, a method for oxidizing monosulfide orpolysulfides electrochemically from an anolyte solution at an anode isdisclosed where the anolyte solution comprises in part an anolytesolvent that dissolves to some extent elemental sulfur. In the method,the anolyte solvent that dissolves to some extent elemental sulfur isone or more of the following: N,N-dimethylaniline, quinoline,tetrahydrofuran, 2-methyl tetrahydrofuran, benzene, cyclohexane,fluorobenzene, thrifluorobenzene, toluene, xylene, tetraethylene glycoldimethyl ether (tetraglyme), diglyme, isopropanol, ethyl propional,dimethyl carbonate, dimethoxy ether, dimethylpropyleneurea, formamide,methyl formamide, dimethyl formamide, acetamide, methyl acetamide,dimethyl acetamide, triethylamine, diethyl acetamide, ethanol and ethylacetate, propylene carbonate, ethylene carbonate, and diethyl carbonate.

In one non-limiting embodiment, the anolyte solvent comprises from about60-100 vol. % polar solvent and 0-40 vol. % apolar solvent. A blend ofdifferent anolyte solvents may help optimize the solubility of elementalsulfur and the solubility of sulfide and polysulfide.

Another non-limiting embodiment discloses a method for removal ofdissolved elemental sulfur from a solvent/alkali metal polysulfidemixture includes cooling, reducing the solubility of sulfur in thesolvent and causing a second liquid phase to form comprising elementalsulfur, and then separating the liquid phase sulfur from the liquidphase solvent mixture. The separation of liquid phase sulfur from liquidphase anolyte includes one or more of the following: gravimetric,centrifugation. The alkali metal polysulfide is of the class includingsodium polysulfide and lithium polysulfide.

The present invention may provide certain advantages, including but notlimited to the following:

Removing an alkali metal continuously or semi-continuously in liquidform from the cell.

Removing sulfur continuously or semi-continuously in liquid form fromthe cell.

Removing high alkali metal polysulfides and dissolved sulfurcontinuously or semi-continuously from the electrolytic cell, therebyreducing polarization of the anode by sulfur.

Separating sulfur continuously or semi-continuously from a streamcontaining a mixture of solvent, sulfur, and alkali metal polysulfidessuch that the solvent and alkali metal polysulfides are substantiallyrecovered such that they can be returned back to an electrolyticprocess.

Operating the electrolytic cells at temperatures and pressures, so thatthe electrolytic cell materials of construction can include materialswhich would not tolerate high elevated temperature.

Reference throughout this specification to features, advantages, orsimilar language does not imply that all of the features and advantagesthat may be realized with the present invention should be or are in anysingle embodiment of the invention. Rather, language referring to thefeatures and advantages is understood to mean that a specific feature,advantage, or characteristic described in connection with an embodimentis included in at least one embodiment of the present invention. Thus,discussion of the features and advantages, and similar language,throughout this specification may, but do not necessarily, refer to thesame embodiment, but may refer to every embodiment.

Furthermore, the described features, advantages, and characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. One skilled in the relevant art will recognize that theinvention may be practiced without one or more of the specific featuresor advantages of a particular embodiment. In other instances, additionalfeatures and advantages may be recognized in certain embodiments thatmay not be present in all embodiments of the invention.

These features and advantages of the present invention will become morefully apparent from the following description and appended claims, ormay be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In order that the manner in which the above-recited and other featuresand advantages of the invention are obtained will be readily understood,a more particular description of the invention briefly described abovewill be rendered by reference to specific embodiments thereof that areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered to be limiting of its scope, the invention will bedescribed and explained with additional specificity and detail throughthe use of the accompanying drawings in which:

FIG. 1 shows an overall process for removing nitrogen, sulfur, and heavymetals from sulfur-, nitrogen-, and metal-bearing oil sources using analkali metal and for regenerating the alkali metal.

FIGS. 2A and 2B show schematic processes for converting alkali metalhydrosulfide to alkali metal polysulfide and recovering hydrogensulfide.

FIG. 3 shows a schematic cross-section of an electrolytic cell whichutilizes many of the features within the scope of the invention.

FIG. 4 shows a schematic of several electrolytic cells operated inseries to extract alkali metal and oxidize alkali metal sulfide topolysulfide and low polysulfide to high polysulfide and high polysulfideto sulfur.

DETAILED DESCRIPTION OF THE INVENTION

The present embodiments of the present invention will be best understoodby reference to the drawings, wherein like parts are designated by likenumerals throughout. It will be readily understood that the componentsof the present invention, as generally described and illustrated in thefigures herein, could be arranged and designed in a wide variety ofdifferent configurations. Thus, the following more detailed descriptionof the embodiments of the methods and cells of the present invention, asrepresented in FIGS. 1 through 4, is not intended to limit the scope ofthe invention, as claimed, but is merely representative of presentembodiments of the invention.

The overall process is shown schematically in FIG. 1 of one non-limitingembodiment for removing nitrogen, sulfur, and heavy metals from sulfur-,nitrogen-, and metal-bearing oil sources using an alkali metal and forregenerating the alkali metal. In the process 100 of FIG. 1, an oilsource 102, such as high-sulfur petroleum oil distillate, crude, heavyoil, bitumen, or shale oil, is introduced into a reaction vessel 104. Analkali metal (M) 106, such as sodium or lithium, is also introduced intothe reaction vessel, together with a quantity of hydrogen 108. Thealkali metal and hydrogen react with the oil and its contaminants todramatically reduce the sulfur, nitrogen, and metal content through theformation of sodium sulfide compounds (sulfide, polysulfide andhydrosulfide) and sodium nitride compounds. Examples of the processesare known in the art, including but not limited to, U.S. Pat. Nos.3,785,965; 3,787,315; 3,788,978; 4,076,613; 5,695,632; 5,935,421; and6,210,564.

The alkali metal (M) and hydrogen react with the oil at about 350° C.and 300-2000 psi according to the following initial reactions:

R—S—R′+2M+H₂→R—H+R′—H+M₂S

R,R′,R″—N+3M+1.5H₂→R—H+R′—H+R″—H+M₃N

Where R, R′, R″ represent portions of organic molecules or organicrings.

The sodium sulfide and sodium nitride products of the foregoingreactions may be further reacted with hydrogen sulfide 110 according tothe following reactions:

M₂S+H₂S→2 MHS (liquid at 375° C.)

M₃N+3H₂S→3 MHS+NH₃

The nitrogen is removed in the form of ammonia 112, which may be ventedand recovered. The sulfur is removed from the oil source in the form ofan alkali hydrosulfide (MHS), such as sodium hydrosulfide (NaHS) orlithium hydrosulfide (LiHS). The reaction products 113, are transferredto a separation vessel 114. Within the separation vessel 114, the heavymetals 116 and upgraded oil organic phase 118 may be separated bygravimetric separation techniques.

The alkali hydrosulfide (MHS) is separated for further processing. Thealkali hydrosulfide stream may be the primary source of alkali metal andsulfur from the process of the present invention. When the alkalihydrosulfide is reacted with a medium to high polysulfide (i.e. M₂S_(x);4≦x≦6) then hydrogen sulfide will be released and the resulting mixturewill have additional alkali metal and sulfide content where the sulfurto alkali metal ratio is lower. The hydrogen sulfide 110 can be used inthe washing step upstream where alkali sulfide and alkali nitride andmetals need to be removed from the initially treated oil.

A schematic representation of this process is shown in FIG. 2A. Forexample, in the case of sodium the following reaction may occur:

Na₂S_(x)+2NaHS→H₂S+2[Na₂S_((x+1)/2)]

Where x:y represent the average ratio of sodium to sulfur atoms in thesolution. In the process shown in FIG. 2A, an alkali polysulfide withhigh sulfur content is converted to an alkali polysulfide with a lowersulfur content.

Alternatively, rather than reacting the alkali metal hydrosulfide withan alkali metal polysulfide, the alkali metal hydrosulfide can bereacted with sulfur. A schematic representation of this process is shownin FIG. 2B. For example, in the case of sodium the following reactionmay occur:

YS+2NaHS→H₂S+Na₂S_((Y+1))

Where Y is a molar amount of sulfur added to the sodium hydrosulfide.

The alkali metal polysulfide may be further processed in an electrolyticcell to remove and recover sulfur and to remove and recover the alkalimetal. One electrolytic cell 120 is shown in FIG. 1. The electrolyticcell 120 receives alkali polysulfide 122. Under the influence of asource electric power 124, alkali metal ions are reduced to form thealkali metal (M) 126, which may be recovered and used as a source ofalkali metal 106. Sulfur 128 is also recovered from the process of theelectrolytic cell 120. A detailed discussion of one possibleelectrolytic cell that may be used in the process within the scope ofthe present invention is given with respect to FIG. 3. A more detaileddiscussion relating to the recovery of sulfur 128 is given with respectto FIG. 4, below.

The vessel where the reaction depicted in FIGS. 2A and 2B occurs couldbe the anolyte compartment of the electrolytic cell 120 depicted in FIG.1, the thickener 312 depicted in FIG. 4, or in a separate vesselconducive to capturing and recovering the hydrogen sulfide gas 110generated. Alternatively, sulfur generated in the process depicted inFIG. 1 could be used as an input as depicted in FIG. 2B.

FIG. 3 shows a schematic sectional view of an electrolytic cell 300which utilizes many of the features within the scope of the invention.The cell is comprised of a housing 310, which typically is an electricalinsulator and which is chemically resistant to solvents and sodiumsulfide. A cation conductive membrane 312, in this case in the form of atube, divides the catholyte compartment 314 from the anolyte compartment316. Within the catholyte compartment is a cathode 324. The cathode 324may be configured to penetrate the housing 310 or have a lead 326 thatpenetrates the housing 310 so that a connection may be made to negativepole of a DC electrical power supply (not shown). Within the anolytecompartment 316 is an anode 326 which in this case is shown as a porousmesh type electrode in a cylindrical form which encircles the membranetube 312. A lead 328 penetrates the housing so that a connection may bemade with a positive pole of the DC power supply. An anolyte solutionflows through an anolyte inlet 330. The anolyte is comprised of amixture of solvents and alkali metal sulfides. As anolyte flows inthrough the inlet 330 anolyte also flows out of the outlet 332. In somecases a second liquid phase of molten sulfur may also exit with theanolyte. A second outlet may be provided from the anolyte compartment ata location lower than the anolyte outlet 332. The second, lower outletmay be used more for removal of molten sulfur that has settled andaccumulated at the cell bottom. The space between the cathode 324 andthe membrane 312 is generally filled with molten alkali metal. As thecell operates, alkali metal ions pass through the membrane 312 andreduce at the cathode 324 to form alkali metal in the catholytecompartment 314 resulting in a flow of alkali metal through thecatholyte outlet 334.

A cell may have multiple anodes, cathodes, and membranes. Within a cellthe anodes all would be in parallel and the cathodes all in parallel.

Referring to FIG. 3, electrolytic cell housing 310 is preferably anelectrically insulative material such as most polymers. The materialalso is preferably chemically resistant to solvents.Polytetrafluoroethylene (PTFE) is particularly suitable, as well asKynar® polyvinylidene fluoride, or high density polyethylene (HDPE). Thecell housing 310 may also be fabricated from a non insulative materialand non-chemically resistant materials, provided the interior of thehousing 310 is lined with such an insulative and chemically resistantmaterial. Other suitable materials would be inorganic materials such asalumina, silica, alumino-silicate and other insulative refractory orceramic materials.

The cation conductive membrane 312 preferably is substantially permeableonly to cations and substantially impermeable to anions, polyanions, anddissolved sulfur. The membrane 312 may be fabricated in part from analkali metal ion conductive material. If the metal to be recovered bythe cell is sodium, a particularly well suited material for the divideris known as NaSICON which has relatively high ionic conductivity at roomtemperature. A typical NaSICON composition substantially would beNa_(1+x)Zr₂Si_(x)P_(3−x)O₁₂ where 0<x<3. Other NaSICON compositions areknown in the art. Alternatively, if the metal to be recovered in thecell is lithium, then a particularly well suited material for thedivider would be lithium titanium phosphate (LTP) with a compositionthat is substantially, Li_((1+x+4y))Al_(x)Ti_((1−x−y))(PO₄)₃ where0<x<0.4, 0<y<0.2. Other suitable materials may be from the ionicallyconductive glass and glass ceramic families such as the generalcomposition Li_(1+x)Al_(x)Ge_(2−x)PO₄. Other lithium conductivematerials are known in the art. The membrane 312 may have a portion ofits thickness which has negligible through porosity such that liquids inthe anolyte compartment 316 and catholyte compartment 314 cannot passfrom one compartment to the other but substantially only alkali ions(M⁺), such as sodium ions or lithium ions, can pass from the anolytecompartment 316 to the catholyte compartment 314. The membrane may alsobe comprised in part by an alkali metal conductive glass-ceramic such asthe materials produced by Ohara Glass of Japan.

The anode 326 is located within the anolyte compartment 316. It may befabricated from an electrically conductive material such as stainlesssteel, nickel, iron, iron alloys, nickel alloys, and other anodematerials known in the art. The anode 326 is connected to the positiveterminal of a direct current power supply. The anode 326 may be a mesh,monolithic structure or may be a monolith with features to allow passageof anolyte through the anode structure. Anolyte solution is fed into theanolyte compartment through an inlet 330 and passes out of thecompartment through and outlet 332. The electrolytic cell 300 can alsobe operated in a semi-continuous fashion where the anolyte compartmentis fed and partially drained through the same passage.

The electronically conductive cathode 324 is in the form of a strip,band, rod, or mesh. The cathode 324 may be comprised of most electronicconductors such as steel, iron, copper, or graphite. A portion of thecathode may be disposed within the catholyte compartment 314 and aportion outside the catholyte compartment 314 and cell housing 310 forelectrical contact. Alternatively, a lead 325 may extend from thecathode outside the cell housing 310 for electrical contact.

Within the catholyte compartment 314 is an alkali ion conductive liquidwhich may include a polar solvent. Non-limiting examples of suitablepolar solvents are as tetraethylene glycol dimethyl ether (tetraglyme),diglyme, dimethyl carbonate, dimethoxy ether, propylene carbonate,ethylene carbonate, diethyl carbonate and such. An appropriate alkalimetal salt, such as a chloride, bromide, iodide, perchlorate,hexafluorophosphate or such, is dissolved in the polar solvent to formthat catholyte solution. Most often the catholyte is a bath of moltenalkali metal.

One non-limiting example of the operation of the electrolytic cell 300is described as follows: Anolyte solution is fed into the anolytecompartment 316. The electrodes 324, 326 are energized such that thereis an electrical potential between the anode 326 and the cathode 324that is greater than the decomposition voltage which ranges betweenabout 1.8V and about 2.5V depending on the composition. Concurrently,alkali metal ions, such as sodium ions, pass through the membrane 312into the catholyte compartment 314, sodium ions are reduced to themetallic state within the catholyte compartment 314 with electronssupplied through the cathode 324, and sulfide and polysulfide isoxidized at the anode 326 such that low polysulfide anions become highpolysulfide anions and/or elemental sulfur forms at the anode. Whilesulfur is formed it is dissolved into the anolyte solvent in entirety orin part. On sulfur saturation or upon cooling, sulfur may form a secondliquid phase of that settles to the bottom of the anolyte compartment316 of the electrolytic cell. The sulfur may be removed with the anolytesolution to settle in a vessel outside of the cell or it may be directlyremoved from a settling zone 336 via an optional sulfur outlet 338, asshown in FIG. 3.

A mode of operation may be to have the anolyte of one electrolytic cellflow into a second cell and from a second cell into a third cell, and soforth where in each successive cell the ratio of sodium to sulfidedecreases as the polysulfide forms become of higher order. FIG. 4 isnon-limiting schematic of four electrolytic cells, 402, 404, 406, 408operated in series to extract alkali metal and oxidize alkali metalsulfide to low alkali metal polysulfide, to oxide low alkali metalpolysulfide to higher alkali metal polysulfide, and to oxide higheralkali metal polysulfide to high alkali metal polysulfide, and to oxidehigh alkali metal polysulfide to sulfur. The electrolytic cells 402,404, 406, and 408 may be operated such that only in the final cell issulfur produced but where alkali metal is produced at the cathode of allof them.

In a non-limiting example, an alkali metal monosulfide, such as sodiumsulfide (Na₂S) may be introduced into the first electrolytic cell 402.Under the influence of a DC power supply, sodium ions are transportedfrom the anolyte compartment to the catholyte compartment where thealkali ions are reduced to form alkali metal. Sulfide is oxidized in theanolyte compartment to form a low polysulfide, such as Na₂S₄. The lowalkali metal polysulfide is transported to the anolyte compartment of asecond electrolytic cell 404. Under the influence of a DC power supply,sodium ions are transported from the anolyte compartment to thecatholyte compartment where the alkali ions are reduced to form alkalimetal. The low polysulfide is oxidized in the anolyte compartment toform a higher polysulfide, such as Na₂S₆. The higher alkali metalpolysulfide is transported to the anolyte compartment of a thirdelectrolytic cell 406. Under the influence of a DC power supply, sodiumions are transported from the anolyte compartment to the catholytecompartment where the alkali ions are reduced to form alkali metal. Thehigher polysulfide is oxidized in the anolyte compartment to form a highpolysulfide, such as Na₂S₈. The high alkali metal polysulfide istransported to the anolyte compartment of a fourth electrolytic cell408. Under the influence of a DC power supply, sodium ions aretransported from the anolyte compartment to the catholyte compartmentwhere the alkali ions are reduced to form alkali metal. High polysulfideis oxidized in the anolyte compartment to form sulfur, which issubsequently removed from the anolyte compartment and recovered.

It will be understood that the foregoing examples of differentpolysulfides are given as representative examples of the underlyingprinciple that that higher order polysulfides may be formed by and thecombination of oxidizing the polysulfide and removing sodium ions.

The multi-cell embodiment described in relation to FIG. 4 enables alkalimetal and sulfur to be formed more energy efficiently compared to asingle cell embodiment. The reason for the energy efficiency is becauseit requires less energy to oxidize lower polysulfides compared to higherpolysulfides. The voltage required to oxidize polysulfides to sulfur isabout 2.2 volts, whereas monosulfide and low polysulfide may be oxidizedat a lower voltage, such as 1.7 volts.

In the case of the alkali metal being sodium, the following typicalreactions may occur in the electrolytic cell 300:

At the Cathode:

Na⁺+e−→Na

At the Anode:

Na₂S_(x)→Na⁺+e⁻+½ Na₂S_((2x))   1)

Na₂S_(x)→Na⁺+e⁻+½ Na₂S_(x)+x/16 S₈   2)

Where x ranges from 0 to about 8.

Most sodium is produced commercially from electrolysis of sodiumchloride in molten salt rather than sodium polysulfide, but thedecomposition voltage and energy requirement is about half forpolysulfide compared to chloride as shown in Table 1.

TABLE 1 Decomposition voltage and energy (watt-hour/mole) of sodium andlithium chlorides and sulfides NaCl Na₂S LiCl Li₂S V 4.0 <2.1 4.2 2.3Wh/mole 107 <56 114 60

The open circuit potential of a sodium/polysulfide cell is as low as1.8V when a lower polysulfide, Na₂S₃ is decomposed, while the voltagerises with rising sulfur content. Thus, it may be desirable to operate aportion of the electrolysis using anolyte with lower sulfur content. Inone embodiment, a planar NaSICON or Lithium Titanium Phosphate (LTP)membrane is used to regenerate sodium or lithium, respectively. NaSICONand LTP have good low temperature conductivity as shown in Table 2. Theconductivity values for beta alumina were estimated from the 300° C.conductivity and activation energy reported by May. G. May, J. PowerSources, 3, 1 (1978).

TABLE 2 Conductivities of NaSICON, LTP, Beta alumina at 25° C., 120° C.Conductivity mS/cm Temperature ° C. NaSICON LTP Beta alumina (est) 250.9 0.9 0.7 120 6.2 1.5 7.9

It may be beneficial to operate 2 or more sets of cells, a non-limitingexample of which is shown in FIG. 4. Some cells would operate with lowerorder sulfide and polysulfides in the anolyte while another set of cellsoperate with higher order polysulfide. In the latter, free sulfur wouldbecome a product requiring removal.

The following example is provided below which discusses one specificembodiment within the scope of the invention. This embodiment isexemplary in nature and should not be construed to limit the scope ofthe invention in any way.

An electrolytic flow cell utilizes a 1″ diameter NaSICON membrane withapproximately 3.2 cm² active area. The NaSICON is sealed to a scaffoldcomprised of a non-conductive material that is also tolerant of theenvironment. One suitable scaffold material is alumina. Glass may beused as the seal material. The flow path of electrolytes will be througha gap between electrodes and the membrane. The anode (sulfur electrode)may be comprised of aluminum. The cathode may be either aluminum orstainless steel. It is within the scope of the invention to configurethe flow cell with a bipolar electrodes design. Anolyte and catholytesolutions will each have a reservoir and pump. The anolyte reservoirwill have an agitator. The entire system will preferably havetemperature control with a maximum temperature of 150° C. and also beconfigured to be bathed in a dry cover gas. The system preferably willalso have a power supply capable of delivering to 5 VDC and up to 100mA/cm².

As much as possible, materials will be selected for construction thatare corrosion resistant with the expected conditions. The flow cell willbe designed such that the gap between electrodes and membrane can bevaried.

In view of the foregoing, it will be appreciated that the disclosedinvention includes one or more of the following advantages:

Removing an alkali metal continuously or semi-continuously in liquidform from the cell.

Removing sulfur continuously or semi-continuously in liquid form fromthe cell.

Removing high alkali metal polysulfides and dissolved sulfurcontinuously or semi-continuously from the electrolytic cell, therebyreducing polarization of the anode by sulfur.

Separating sulfur continuously or semi-continuously from a streamcontaining a mixture of solvent, sulfur, and alkali metal polysulfidessuch that the solvent and alkali metal polysulfides are substantiallyrecovered such that they can be returned back to an electrolyticprocess.

Operating the electrolytic cells at temperatures and pressures, so thatthe electrolytic cell materials of construction can include materialswhich would not tolerate high elevated temperature.

While specific embodiments of the present invention have beenillustrated and described, numerous modifications come to mind withoutsignificantly departing from the spirit of the invention, and the scopeof protection is only limited by the scope of the accompanying claims.

1. A process for oxidizing alkali metal monosulfides and polysulfideselectrochemically comprising: obtaining an electrolytic cell comprisingan alkali ion conductive membrane configured to selectively transportalkali ions, the membrane separating an anolyte compartment configuredwith an anode and a catholyte compartment configured with a cathode;introducing into the anolyte compartment an anolyte solution comprisingan alkali metal monosulfide, an alkali metal polysulfide, or a mixturethereof and an anolyte solvent that partially dissolves elementalsulfur; introducing into the catholyte compartment a catholyte whereinthe catholyte comprises a molten alkali metal; applying an electriccurrent to the electrolytic cell at an operating temperature thereby: i.oxidizing monosulfide or polysulfide in the anolyte compartment to formliquid elemental sulfur; ii. causing alkali metal ions to pass throughthe alkali ion conductive membrane from the anolyte compartment to thecatholyte compartment; and iii. reducing the alkali metal ions in thecatholyte compartment to form liquid elemental alkali metal; allowingliquid elemental sulfur to become saturated in the anolyte solution andto form a second liquid phase.
 2. The process according to claim 1 wherethe liquid elemental sulfur separates from the anolyte solution in asettling zone that is within the electrolytic cell.
 3. The processaccording to claim 1 where the liquid elemental sulfur separates fromthe anolyte solution in a settling zone that is external to the cell. 4.The process according to claim 1 where the separation of liquidelemental sulfur from the anolyte solution includes one or more of theseparation techniques selected from gravimetric, filtration, andcentrifugation.
 5. The process according to claim 1, wherein the alkaliion conductive membrane is substantially impermeable to anions, thecatholyte solvent, the anolyte solvent, and dissolved sulfur.
 6. Theprocess according to claim 1, wherein the alkali ion conductive membranecomprises in part an alkali metal conductive ceramic or glass ceramic.7. The process according to claim 1, wherein the alkali ion conductivemembrane comprises a solid MSICON (Metal Super Ion CONducting) material,where M is Na or Li.
 8. The process according to claim 1, wherein theanolyte solvent comprises one or more solvents selected fromN,N-dimethylaniline, quinoline, tetrahydrofuran, 2-methyltetrahydrofuran, benzene, cyclohexane, fluorobenzene, thrifluorobenzene,toluene, xylene, tetraethylene glycol dimethyl ether (tetraglyme),diglyme, isopropanol, ethyl propional, dimethyl carbonate, dimethoxyether, dimethylpropyleneurea, formamide, methyl formamide, dimethylformamide, acetamide, methyl acetamide, dimethyl acetamide,triethylamine, diethyl acetamide, ethanol and ethyl acetate, propylenecarbonate, ethylene carbonate, and diethyl carbonate.
 9. The processaccording to claim 1, wherein the anolyte solvent comprises from about60-100 vol. % polar solvent and 0-40 vol. % apolar solvent.
 10. Aprocess for oxidizing alkali metal polysulfides and monosulfideselectrochemically comprising: obtaining a first electrolytic cellcomprising an alkali ion conductive membrane configured to selectivelytransport alkali ions, the membrane separating an anolyte compartmentconfigured with an anode and a catholyte compartment configured with acathode; introducing into the anolyte compartment an anolyte solutioncomprising an alkali metal monosulfide, an alkali metal polysulfide, ora mixture thereof and an anolyte solvent that partially dissolveselemental sulfur; introducing into the catholyte compartment acatholyte, wherein the catholyte comprises a molten alkali metal;applying an electric current to the electrolytic cell thereby: i.oxidizing sulfide or polysulfide in the anolyte compartment to form ahigher level polysulfide; ii. causing alkali metal ions to pass throughthe alkali ion conductive membrane from the anolyte compartment to thecatholyte compartment; and iii. reducing the alkali metal ions in thecatholyte compartment to form elemental alkali metal; transportinganolyte solution from the first electrolytic cell to a secondelectrolytic cell comprising an alkali ion conductive membraneconfigured to selectively transport alkali ions, the membrane separatingan anolyte compartment configured with an anode and a catholytecompartment configured with a cathode and a catholyte; applying anelectric current to the second electrolytic cell thereby: i. oxidizingpolysulfide in the anolyte compartment to form liquid elemental sulfur;ii. causing alkali metal ions to pass through the alkali ion conductivemembrane from the anolyte compartment to the catholyte compartment; andiii. reducing the alkali metal ions in the catholyte compartment to formliquid elemental alkali metal; allowing liquid elemental sulfur tobecome saturated in the anolyte solution and to form a second liquidphase.
 11. The process according to claim 10 where the liquid elementalsulfur separates from the anolyte solution in a settling zone that iswithin the electrolytic cell.
 12. The process according to claim 10where the liquid elemental sulfur separates from the anolyte solution ina settling zone that is external to the cell.
 13. The process accordingto claim 10 where the separation of liquid elemental sulfur from theanolyte solution includes one or more of the separation techniquesselected from gravimetric, filtration, and centrifugation.
 14. Theprocess according to claim 10, wherein the alkali ion conductivemembrane is substantially impermeable to anions, the catholyte solvent,the anolyte solvent, and dissolved sulfur.
 15. The process according toclaim 10, wherein the alkali ion conductive membrane comprises in partan alkali metal conductive ceramic or glass ceramic.
 16. The processaccording to claim 10, wherein the alkali ion conductive membranecomprises a solid MSICON (Metal Super Ion CONducting) material, where Mis Na or Li.
 17. The process according to claim 10, wherein the anolytesolvent comprises one or more solvents selected fromN,N-dimethylaniline, quinoline, tetrahydrofuran, 2-methyltetrahydrofuran, benzene, cyclohexane, fluorobenzene, thrifluorobenzene,toluene, xylene, tetraethylene glycol dimethyl ether (tetraglyme),diglyme, isopropanol, ethyl propional, dimethyl carbonate, dimethoxyether, dimethylpropyleneurea, formamide, methyl formamide, dimethylformamide, acetamide, methyl acetamide, dimethyl acetamide,triethylamine, diethyl acetamide, ethanol and ethyl acetate, propylenecarbonate, ethylene carbonate, and diethyl carbonate.
 18. The processaccording to claim 10, wherein the anolyte solvent comprises from about60-100 vol. % polar solvent and 0-40 vol. % apolar solvent.
 19. Anelectrolytic cell for oxidizing alkali metal polysulfides comprising: ananolyte compartment configured with an anode and containing an anolytesolution comprising an alkali metal monosulfide, an alkali metalpolysulfide, or a mixture thereof and an anolyte solvent that partiallydissolves elemental sulfur, the anolyte compartment further comprisingan anolyte solution inlet and an anolyte solution outlet; a catholytecompartment configured with a cathode and containing a catholyte,wherein the catholyte comprises a molten alkali metal, the catholytecompartment further comprising a catholyte outlet; an alkali ionconductive membrane configured to selectively transport alkali ions,wherein the alkali ion conductive membrane is substantially impermeableto anions, the anolyte solvent, and dissolved sulfur; a source ofelectric potential electrically coupled to the anode and the cathode andconfigured to: oxidize monosulfide or polysulfide in the anolytecompartment to form liquid elemental sulfur; cause alkali metal ions topass through the alkali ion conductive membrane from the anolytecompartment to the catholyte compartment; and reduce the alkali metalions in the catholyte compartment to form liquid elemental alkali metal;and an elemental sulfur settling zone where liquid elemental sulfurseparates from the anolyte solution.
 20. The electrolytic cell accordingto claim 19, wherein the alkali ion conductive membrane comprises inpart an alkali metal conductive ceramic or glass ceramic.
 21. Theelectrolytic cell according to claim 19, wherein the alkali ionconductive membrane comprises a solid MSICON (Metal Super IonCONducting) material, where M is Na or Li.
 22. The electrolytic cellaccording to claim 19, wherein the anolyte solvent comprises one or moresolvents selected from N,N-dimethylaniline, quinoline, tetrahydrofuran,2-methyl tetrahydrofuran, benzene, cyclohexane, fluorobenzene,thrifluorobenzene, toluene, xylene, tetraethylene glycol dimethyl ether(tetraglyme), diglyme, isopropanol, ethyl propional, dimethyl carbonate,dimethoxy ether, dimethylpropyleneurea, formamide, methyl formamide,dimethyl formamide, acetamide, methyl acetamide, dimethyl acetamide,triethylamine, diethyl acetamide, ethanol and ethyl acetate, propylenecarbonate, ethylene carbonate, and diethyl carbonate.
 23. Theelectrolytic cell according to claim 19, wherein the anolyte solventcomprises from about 60-100 vol. % polar solvent and 0-40 vol. % apolarsolvent.
 24. The electrolytic cell according to claim 19, furthercomprising a sulfur outlet for removal of elemental sulfur from theelectrolytic cell.